Recently, solar power has attracted attention as a typical clean energy. Backsheets disposed on the backmost surfaces of solar modules are members for protecting a generation mechanism including cells and wiring from the external environment to maintain insulation, and are composed of various functional films laminated with an adhesive. The adhesive used for such backsheets requires high adhesion to various films with different characteristics, such as polyester films and polyvinyl fluoride films, and a high level of heat and moisture resistance sufficient to maintain adhesion for an extended period of time in outdoor environments.
One known adhesive for backsheets is a two-part polyurethane adhesive containing, as a main agent, a polyester polyurethane polyol prepared by reacting a polyester polyol with isophorone diisocyanate and, as a curing agent, isophorone diisocyanate trimer (see PTL 1). The polyester polyol is prepared by reacting neopentyl glycol, 1,6-hexanediol, ethylene glycol, isophthalic acid, and sebacic acid. However, since such an adhesive containing a polyester polyurethane polyol having no branched structure has low crosslink density when finally cured, the cured adhesive swells easily and exhibits decreased adhesion under hot and humid conditions. Thus, this adhesive has low adhesion to substrates under hot and humid conditions.